Optional function/method parameters incorrectly formatted

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Originally filed by @jeffque as google/quiver-dart#575:

The documentation for the function quiver.core.firstNonNull is being displayed like this:

firstNonNull(dynamic o1, [ dynamic o2, [ dynamic o3 dynamic o4 ])

with an extra square brakets between o1 and o2. I also noted a lack of comma separating o3 and o4 now. Maybe related?

It can be found at least here and here

But, seeing the code, I would expect something like this

firstNonNull(dynamic o1, dynamic o2, [ dynamic o3, dynamic o4 ])

or this:

firstNonNull(o1, o2, [ o3, o4 ])

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Thanks for the fix!